1. Field of the Invention
This invention relates to an optical scanner and also to an image forming apparatus. More particularly, the present invention relates to an optical scanner comprising a scanning optical system including a diffracting section and a refracting section. Such an optical scanner can suitably be used for a laser beam printer (LBP) or a digital copying machine designed to form an image of an object.
2. Related Background Art
Optical scanners to be used for image-forming apparatus including laser beam printers and digital copying machines are adapted to deflect a light beam emitted from a light source in accordance with an image signal by means of an optical deflector, converge the deflected light beam to a spot of light on the surface to be scanned of a photosensitive drum by means of a scanning optical system and cause the light beam to scan the surface in order to form an image thereon.
Additionally, optical scanners comprising a scanning optical system having a diffracting section at least on a plane thereof are known. For examples, such an optical scanner is proposed in Japanese Patent Application Laid-Open No. 10-68903. According to the above patent document, a scanning optical element having a refracting section and a diffracting section is used for the scanning optical system. Fluctuations of the magnification and the focal point in the main-scanning direction due to the temperature fluctuations of the optical scanner can be corrected by changing the power of the refracting section and that of the diffracting section of the scanning optical element along with the wavelength of the semiconductor laser of the light source provided that the power of the refracting section and that of the diffracting section are so selected as to satisfy certain requirements. With this arrangement, finely defined images can be obtained even when the ambient temperature fluctuates.
For a scanning optical system having a diffracting section to generate necessary power by diffraction, the pitch of the diffraction grating 20 have to be varied in a manner as shown in FIG. 1 of the accompanying drawings. For instance, a convex power or a concave power can be realized by making the pitch of the diffraction grating 20 vary as a function of the distance from the optical axis. The diffracting section proposed in the above Japanese Patent Application Laid-Open No. 10-68903 is designed to generate a convex power by making the pitch of the diffraction grating diminish as a function of the distance from the optical axis in the main-scanning direction in order to achieve a desired imaging performance.
Generally, as the pitch of diffraction grating is reduced, so increases the intensity of diffracted light of the unnecessary orders of diffraction relative to that of diffracted light of the necessary orders of diffraction. This means that the quantity of diffracted light of the unnecessary orders of diffraction increases and hence that of diffracted light of the necessary orders of diffraction decreases. Then, as a matter of course, after passing through the diffracting section, the quantity of light of diffracted light of the necessary orders of diffraction decreases relative to the quantity of light entering the diffracting section. Therefore, when causing the scanning optical system to generate power by diffraction in a manner as described above, its diffraction efficiency of the diffracting section decreases as a function of the distance from the optical axis so that consequently the illuminance distribution to which the diffracting section is responsible is reduced.
Another factor that affects the illuminance distribution is the surface reflection of the various optical planes of the scanning optical system. FIG. 2 is a graph illustrating the angle dependency of the reflectance and the transmittance of an optical member showing a refractive index of 1.525 when an S-polarized light beam is made to enter the optical member. The surface reflection of each of the optical planes increases as the angle of incidence increases. Therefore, because the angle of incidence generally increases as a function of the distance from the optical axis of a scanning optical means, the surface reflection also increases as a function of the distance to consequently lower the transmittance. Thus, when an S-polarized light beam is made to enter an scanning optical means having a diffracting section, both the transmittance and the diffraction efficiency falls as a function of the distance from the optical axis of the scanning optical means so that the illuminance distribution falls on the surface to be scanned also as a function of the distance. As a result, the output image is accompanied by the drawback that it is less dense in peripheral areas than in central areas.
In view of the above problem of the prior art, it is therefore the object of the present invention to provide an optical scanner that can provide a uniform illuminance distribution on the surface to be scanned so that it can output a high quality image if the scanning optical system of the optical scanner has a diffracting section and also provide an image forming apparatus comprising such an optical scanner.
According to the invention, the above object is achieved by providing an optical scanner comprising:
a light source;
an optical deflector for deflecting the light beam emitted from the light source; and
a scanning optical system for focussing the light beam deflected by the optical deflector on a surface to be scanned, the scanning optical system having a diffracting section and a refracting section;
the illuminance distribution of the light beam being maintained to be substantially uniform on the surface to be scanned by offsetting the change in the diffraction efficiency of the diffracting section due to the angle of view of the scanning optical system and the change in the transmittance of the refracting section due to the angle of view of the scanning optical system.
In another aspect of the invention, there is also provided an optical scanner comprising:
a light source;
an optical deflector for deflecting the light beam emitted from the light source; and
a scanning optical system for focussing the light beam deflected by the optical deflector on a surface to be scanned, the scanning optical system having a diffracting section, a refracting section and at least a reflector mirror;
the illuminance distribution of the light beam being maintained to be substantially uniform on the surface to be scanned by offsetting the change in the diffraction efficiency of the diffracting section due to the angle of view of the scanning optical system, the change in the transmittance of the refracting section due to the angle of view of the scanning optical system and the change in the reflectance of the reflector mirror due to the angle of view of the scanning optical system.
In still another aspect of the invention, there is also provided an optical scanner comprising:
a light source;
an optical deflector for deflecting the light beam emitted from the light source; and
a scanning optical system for focussing the light beam deflected by the optical deflector on a surface to be scanned, the scanning optical system having a diffracting section, a refracting section and at least an optical filter;
the illuminance distribution of the light beam being maintained to be substantially uniform on the surface to be scanned by offsetting the change in the diffraction efficiency of the diffracting section due to the angle of view of the scanning optical system, the change in the transmittance of the refracting section due to the angle of view of the scanning optical system and the change in the transmittance of the optical filter due to the angle of view of the scanning optical system.